Mice infected with the neurotropic JHM strain of the murine coronavirus, mouse hepatitis virus, (JHMV) develop acute and chronic demyelinating disease, which are useful models for the human disease, multiple sclerosis. In one version, suckling C57Bl/6 (B6) mice are infected with JHMV and are protected from developing acute encephalitis by nursing by JHMV-immune dams. A variable percentage of mice develop demyelinating encephalomyelitis 3-8 weeks after infection; this infection is characterized by mutations in the immunodominant CD8 T cell epitope (S510) recognized in B6 mice. These mutations abrogate recognition by S510-specific CD8 T cells (cytotoxic lymphocyte escape, CTL escape). A second epitope (S598) is also recognized in these mice, but the immune response to this epitope does not prevent CTL escape. However, modification of the S598 peptide that result in better binding to MHC class I antigen results in a CD8 T cell response with higher functional avidity. Remarkably, immunization with this modified epitope now selects for CD8 T cells that more potently recognize the native S598 epitope than do cells elicited by the original epitope, thereby exhibiting a heteroclitic effect. Immunization with the modified epitope protects against CTL escape. The main goal of this proposal is to probe why an epitope is heteroclitic. The first specific aim will evaluate the efficacy of the CD8 T cell response induced by the previously identified heteroclitic S598 epitope in diminishing demyelination after infection with an attenuated strain of JHMV. The second specific aim will identify other heteroclitic variants of S598 and to begin to identify features that facilitate prediction of such heteroclitic peptides; now heteroclitic epitopes are identified empirically. The third aim will perform structural and biophysical studies of S598-specific T cell receptors (TCRs) in complex with the native or heteroclitic S598 peptide bound to MHCI (H-2Kb). The ultimate goal is to understand the molecular basis of interactions between high affinity TCRs and heteroclitic epitope/MHC class I antigen. By understanding these interactions, it will be possible to design variant epitopes that induce CD8 T cells with high functional avidity that potently cross-react with the native epitope, but do not induce a large fraction of cells with unwanted specificity. This approach will be useful in design of vaccines that are directed at enhancing pathogen-specific CD8 T cell responses.